Intravenous tubing sets are commonly used with an infusion pump to deliver fluid medications directly into the bloodstream of a patient. Fluid, such as glucose or saline, is mixed or dissolved with a medical substance in a bag, and a delivery tube conducts the medicinal fluid from the bag into a needle or other connector which delivers the fluid into a vessel, typically a vein, of the patient. The bag is suspended above the patient so that the force of gravity acting on the fluid causes the fluid to flow through the delivery tube.
A ramp valve and a drip chamber are sometimes included in-line with the delivery tube to control the flow rate of the fluid. Although the ramp valve is useful and sufficiently accurate for some applications, it is not a fully reliable metering device. Variations in the height or head of fluid in the bag cause variations in the fluid flow rate through the delivery tube. Variations in the height of the fluid in the bag occur naturally as the fluid flows from the bag, and also occur when a pole stand which supports the bag above the patient is adjusted in height. In addition, the use of a ramp valve does not typically provide the degree of precision in fluid delivery necessary in many medical situations.
For those medical applications which require precise metering of the medicinal fluid delivered to the patient, infusion pumps are used to precisely control the amount of fluid delivered. A portion of the delivery tube is clamped or connected in the infusion pump, and the infusion pump is operated by electrical power to deliver the fluid at a precise rate. In most cases, infusion pumps are peristaltic in operation. The flexible delivery tube is compressed or pinched at two spaced-apart locations to capture a predetermined volume of fluid in the tube between the pinch points. The pinched locations are mechanically advanced at a predetermined timed rate. This wave-like movement repeats and creates a very precise flow of fluid over time.
When the infusion pump is stopped, the tube remains pinched together by the pumping mechanism to occlude the tube. No fluid can flow to the patient when the pump is stopped and the tube remains within the infusion pump. In some infusion pumps, the peristaltic pinching action occurs with respect to a door on the pump. When the door is closed, the closed door provides a working surface against which the delivery tube is pinched during the peristaltic action. When the door is opened, the delivery tube is no longer pinched because the working surface separates from the delivery tube. Opening the door under these circumstances creates the possibility of the medicinal fluid free-flowing into the patient.
To prevent the fluid from free-flowing when the door is open, infusion pumps typically include an internal occlusion clamp which pinches the delivery tube to occlude the delivery tube. Such internal occlusion clamps become effective when the door is opened. When the door is closed, the internal occlusion clamp is moved to a non-occluding position. Moving the internal occlusion clamp to the non-occluding position when the door is closed allows peristaltic movement of the fluid through the delivery tube by the peristaltic or other internal pumping mechanism.
It is necessary to remove the delivery tube from the infusion pump. Of course to do so, the door must be open to gain access to the delivery tube. Opening the door results in the internal occlusion clamp clamping the delivery tube, making it very difficult or impossible to remove the delivery tube from the infusion pump. Under these circumstances, the infusion pump must include some provision for releasing the internal occlusion clamp even though the door is open. With the delivery tube removed from the infusion pump, there is no restriction on the gravity flow of fluid from the bag through the delivery tube into the patient. Even short times of unrestricted gravity flow can introduce potentially dangerous amounts of the medicinal fluid into the bloodstream of the patient, since the flow rate through the operating infusion pump is typically much less than the gravity-induced free flow rate when the delivery tube is not restricted.
Occasionally when it is necessary to remove the delivery tube from the infusion pump, the tubing set may remain connected to the patient, giving rise to the possibility that the fluid will free-flow by gravity into the patient in an uncontrollable manner. The potential for gravity-induced free flow of fluid into the patient occurs when it is necessary to exchange one infusion pump for a different infusion pump in the case of mechanical failure of an infusion pump. Another circumstance which may give rise to gravity-induced free flow of fluid into the patient is when two bags are connected to a single tubing set and the fluid has been delivered from one of the bags. Under such circumstances, the delivery tube from the empty bag must be removed from the infusion pump and the delivery tube from the full bag substituted within the infusion pump. Under these exemplary circumstances, the fluid may flow from the bag into the patient.
To prevent overdosing the patient when the delivery tubing is removed from the infusion pump and the delivery tubing is still connected to the patient, manual clamps are usually located on the delivery tube external to the infusion pump. Closing such an external clamp prior to removing the tubing from the infusion pump avoids the free flow of fluid into the patient. Such an external clamp may be the ramp valve or a similar roll or slide clamp. However, the success of this procedure depends on the attending medical personnel closing the external clamp. Although unusual, medical personnel may become distracted and fail to close the external clamp. Medical personnel which do not have the necessary level of experience and training may simply fail to recognize the need to close the external clamp when the delivery tube is removed from the infusion pump while the delivery tube remains connected to the patient. Executing the necessary sequence of closing the external clamp before removing the delivery tube from the infusion pump may also increase the risk that relatively inexperienced medical personnel may make a mistake in the sequence of actions required.
Because of the potential for human error when using an infusion pump, certain medical standards and accreditation organizations require that infusion pumps have built-in, automatic anti-free flow capabilities. Such anti-free flow capabilities prevent the free flow of fluid through the delivery tube when the delivery tube is removed from the infusion pump or when the door to the pump is open and the peristaltic constrictions can no longer occlude the delivery tube. Such anti-free flow capability is intended to overcome the natural possibility of inadvertent human failure to clamp the delivery tube externally whenever the tubing set is removed from the infusion pump or whenever the infusion pump is manipulated in such a way that free flow through the delivery tube becomes possible.
A number of previous automatic anti-free flow mechanisms have been employed in infusion pumps. Some of those anti-free flow mechanisms are subject to unintended manipulation in such a way that the delivery tube can be removed from the infusion pump without occluding the delivery tube, thereby defeating the anti-free flow capability. Other anti-free flow mechanisms are more reliable. Regardless of the level of success in preventing gravity flow, almost all of the previous infusion pumps which possess anti-free flow capability are of considerably increased cost and complexity. The increased complexity has also increased the risk of mechanical failure, with the attendant possibility of downtime of the equipment leading to the unavailability of the equipment. The increased complexity also creates an increased risk of successfully defeating the anti-free flow capability, out of a lack of knowledge of the proper functionality of the system or from good faith attempts to remove the delivery tube under appropriate circumstances but using inappropriate techniques.
One such popular infusion pump which incorporates anti-free flow capabilities requires a slide clamp to be inserted into the pump whenever the delivery tube is loaded into the pump. The slide clamp is captured by the internal operating mechanism of the infusion pump when the door is closed, and the slide clamp becomes part of the occlusion clamp operating mechanism. Sensors within the pump detect the presence of the slide clamp as a condition for operating the pump. A complicated arrangement of moving parts within the pump moves the slide clamp relative to the delivery tube to open the delivery tube to the flow of fluid as a part of operating the internal occlusion clamp. The delivery tube is occluded when the delivery tube is forced into a narrow portion of a V-shaped notch in the slide clamp, thereby pinching the side walls of the tube together. The delivery tube is opened to the flow of fluid when the slide clamp is moved in the opposite direction where the delivery tube extends through the wide portion of the V-shaped notch.
A variety of other types of anti-free flow mechanisms are also used in infusion pumps, but these other types of anti-free flow mechanisms do not employ slide clamps. These other types of pumps and anti-free flow mechanisms have had varying degrees of success in preventing inadvertent and intentional human action from overriding the intended anti-free flow capability.